1. Field of the Invention
The present invention relates to a micropump, and in particular a micropump working according to a peristaltic pumping principle.
2. Description of the Related Art
Micropumps working according to a peristaltic pumping principle are known from the prior art. The article “Design and simulation of an implantable medical drug delivery system using microelectromechanical systems technology”, by Li Cao et al., Sensors and Actuators, A94 (2001), pages 117 to 125, deals with a peristaltic micropump comprising an inlet, three pumping chambers, three silicon membranes, three normally closed active valves, three piezo-stack actuators of PZT, microchannels between the pumping chambers, and an outlet. The three pumping chambers are of the same size and are etched into a silicon wafer.
From WO 87/07218 a peristaltic micropump is also known, which has three membrane regions in a continuous substrate area. In a supporting layer supporting the substrate and an associated backing layer, a pumping channel is formed that is in connection with a fluid supply. In the pumping channel, in the region of an inlet valve and an outlet valve, a transverse rib is formed on which an associated membrane portion rests in the non-actuated state to close the inlet valve and the outlet valve in the non-actuated state. Between the separately actuatable membrane regions associated with the inlet valve and the outlet valve, the third membrane region, which may also be actuated separately, is arranged. By actuating the third membrane region, the chamber volume between the two valve regions is increased. Thus, by a corresponding timing of the three membrane regions, a peristaltic pumping effect between inlet valve and outlet valve may be achieved. According to WO 87/07218, the actor element consists of a composite of three elements comprising metal membrane, continuous ceramic layer, and segmented electrode arrangement. The ceramic layer has to be polarized in a segmented manner, which is technically difficult. Such a segmented piezo-bending element thus is expensive and allows only small stroke volumes, so that such a pump cannot work in a bubble-tolerant and self-priming manner.
From DE 19719862 A1, a micromembrane pump not working based on the peristaltic principle is known, wherein a pumping membrane adjoining a pumping chamber may be actuated by a piezo-actor. A fluid inlet and a fluid outlet of the pumping chamber are each provided with passive check valves. According to this document, the compression ratio of the micropump, i.e. the ratio of stroke volume of the pumping membrane to overall pumping chamber volume, is adjusted depending on the maximum pressure value depending on the valve geometry and the valve wetting, which is necessary to open the valves, to enable a bubble-tolerant, self-priming operation of the micromembrane pump there.
Apart from the above-mentioned piezo-actors, it would also be possible to realize micropumps using electrostatic actors, wherein electrostatic actors, however, only enable very small strokes. Alternatively, the realization of pneumatic drives would be possible, which, however, necessitates high expenditure regarding external pneumatics as well as the switching valves required for this. decreased by moving the second membrane region also towards the pump body.
Through this construction, the inventive peristaltic micropump enables the realization of bubble-tolerant, self-priming pumps, even if piezo-elements arranged on the membrane are used as piezo-actor. Alternatively, according to the invention, so-called piezo-stacks may also be used as piezo-actors, which are, however, disadvantageous as opposed to piezo-membrane converters in that they are large and expensive, provide problems with respect to the connection technique between stack and membrane and problems with the adjustment of the stacks, and are thus all in all connected with higher expenditure.
In order to ensure that the inventive peristaltic micropump can work in a bubble-tolerant and self-priming manner, it is preferably dimensioned such that the ratio of stroke volume and dead volume is greater than the ratio of delivery pressure (feed pressure) and atmospheric pressure, wherein the stroke volume is the volume displaceable by the pumping membrane, the dead volume is the volume remaining between inlet opening and outlet opening of the micropump, when the pumping membrane is actuated and one of the valves is closed and one is open, the atmospheric pressure is a maximum of about 1050 hPa (worst case consideration), and the delivery pressure is the pressure necessary in the fluid chamber region of the micropump, i.e. in the pressure chamber, to move a liquid/gas interface past a place representing a flow constriction (bottleneck) in the microperistaltic pump, i.e. between the pumping chamber and the passage opening of the first or the second valve, including this passage opening.
If the ratio of stroke volume and dead volume, which may be referred to as compression ratio, satisfies the above condition, it is ensured that the peristaltic micropump works in a bubble-tolerant and self-priming manner. This Pneumatic drives thus represent expensive, costly and space-intensive methods to implement membrane deflection.